1. Field of the Invention
The present invention relates to a liquid crystal display device provided with an image capturing function.
2. Description of the Related Art
A liquid crystal display device is constituted by a glass array substrate including a pixel array unit in which pixels are arranged at respective intersections of a plurality of signal lines and a plurality of scan lines, a glass counter substrate disposed opposite to this array substrate, and a liquid crystal layer retained in a space between the array substrate and the counter substrate.
In recent years, the manufacturing technique for forming thin film transistors (TFTs) made of a polysilicon material on the array substrate has been established, and the process technique for forming driving circuits to drive the signal lines and the scan lines onto the single array substrate together with the pixel array unit has been put to practical use. In this way, the entire liquid crystal display devices become lighter and smaller, and are used in various devices such as cellular telephones or notebook computers.
Incidentally, there is proposed a liquid crystal display device in which a pixel array unit includes a coherent optical sensor for capturing an image (such as Japanese Unexamined Patent Publication No. H8(1996)-272529). In the liquid crystal display device of this type, a capacitor is connected to an optical sensor configured to flow a current corresponding to irradiation intensity, a amount of charge in this capacitor is changed in response to an amount of light received by the optical sensor, image data are acquired by detecting voltages at both terminals of the capacitor, and the image data are captured into a static random access memory (SRAM).
However, the SRAM can only distinguish binary values of “0” and “1”. Accordingly, in order to acquire the image data in multiple tone levels, it is necessary to capture the image for multiple times while varying conditions. As a result, the time required for image capturing is increased, and it is difficult to capture a subject stably.